Onychophora overview

Onychophorans are commonly
known as the velvet worms. They got this name because their skin is
covered in small bumps many of which have a small filament sticking out of the
top.

There are about 120 species, and they range in length from 1.5 - 15 cm. They are found in tropical and sub-tropical forests.

Velvet worm evolutionary relationships

For many years they were thought to be the missing link between the worms and
the Arthropods, because they share characteristics with both. The fossil record
shows that they have changed little in the last 500 million years.

However it
is now believed that their Arthropod-like characteristics are examples of
parallel evolution, and that they are not an Arthropod ancestor. The main
reason for this is that their tracheae form numerous branches near the spiracle
opening, but rarely ever branch again. Also the spiracles appear to be randomly
located on the body surface and have no closing mechanism. This lack of closing
mechanism confines them to the most humid of environments such as the forest floor, leaf litter and under stones. In fact in an ordinary room Peripatus (below) can lose as much as one third of its body weight in moisture in just 4 hours.

It is now believed that they evolved from the nematodes some time during the Cambrian.

Velvet worm body

Velvet worm cuticle never
hardens like Arthropod cuticle, and is not moulted all at one time, but in
patches. On the head they have a pair
of antennae each with an eye at the
base, the eye is similar to Annelid eyes (see Peripatopsis capensis below right).

Their legs are unjointed, operated hydraulically, and end in a pair of claws. The colour of the upper body varies from dark grey, green, brown or red; with the underneath whitish or light coloured.

Their
mouth has a pair of claw-like mandibles and papillae. The papillae are the
exits for a pair of slime glands.

Onychophoran hunting and defensive behaviour

Onychphorans are predators and they disable
their prey by squirting out a glue-like slime
from the slime glands, usually from a distance of 0.5 cm, but for spiders and other dangerous prey they can aim from as far as
4 cm away. They can squirt up to 30 times aiming at the legs and
jaws of dangerous prey, but for less dangerous prey they may squirt only once. Their prey consists of small insects such as termites, crustaceans such as woodlice, and they will also eat larger insects such as crickets when they find them already dead.

The slime rapidly hardens on contact. When the prey is immobilised the onychophora bites through the exoskeleton and injects saliva to digest the
body contents. While waiting for the saliva to work the slime is eaten as it is a protein and so too nutritious to waste. The slime is also used in
defence. Their usual prey is insects, snails and worms.

Velvet worm sex life

Onychophora have, to us, a
rather lacklustre sex life. A male packages his sperm into a bundle called a
spermatophore. Having succeeded in doing this, it appears he feels he has has done
all that can reasonably be expected of him. So then deposits his bundle on the
next onychophoran he meets, whether it is a male, female or juvenile. Then wanders off. If the spermatophore
lands on a mature female the skin beneath the spermatophore dissolves, and the
sperm enters the body cavity, where some eventually migrate to the ovaries
where fertilization occurs.

Males are usually a little smaller than females, and often have fewer legs.